[GlobalGeometryTypeIndex] Rename base() to offset()

As suggested by Martin this renames GlobalGeometryTypeIndex::base() to
the more self-documenting GlobalGeometryTypeIndex::offset(). For
consistency, the patch also renames regular_base() to
regular_offset(). There should not be any backwards compatiblity
problems, as both methods were private until this morning (and
regular_offset() still is).